An induction motor or a reluctance motor is generally used to replace a rare earth permanent magnet motor, in order to reduce costs of a variable speed motor. The induction motor will lower down motor efficiency although it can reduce the costs of the variable speed motor. The reluctance motor requires larger current, which will increase costs of a frequency converter, and thereby the total costs of the variable speed motor and the frequency converter will be increased.
In addition, as shown in FIGS. 1 and 2, there are several air-gap slots 21 arranged on the rotor 20 of the existing reluctance motor. Each air-gap slot 21 has an end part 21a, and a main body part 21b which is immediately adjacent to the end part 21a. As shown in FIGS. 1 and 2, the end part 21a of the air-gap slot 21 and the main body part 21b corresponding to the end part 21a are aligned with each other, which will greatly limit optimization of motor design and will make it impossible to further reduce torque ripple of the reluctance motor.
Non-uniform air gaps are generally used in order to reduce the torque ripple of a motor. Even though the non-uniform air gap can reduce the torque ripple of the motor, it will increase the complexity of a stator or rotor of the motor, and it is hard to measure the air gap of the motor.